Coastal Erosion Processes near sea dikes in Hai Hau district, Vietnam

The erosion problem in Hai Hau coastal district (Vietnam) is the subject of this thesis. The exact processes that drive this ongoing erosion however are not well defined yet, and therefore two main goals are defined. The first goal is to gain more insight into the processes that drive the ongoing erosion in Hai Hau. The second goal is to predict the future developments of the nearshore area with following consequences for the coastal defense system. The most important results are: 1. Large amounts of wave energy coming from north-eastern directions are blocked by the Ba Lat delta. The only waves reaching the northern part of the Hai Hau coast refract around the delta. These refracted waves are lower than the waves in the other parts of Hai Hau. 2. Modeling south-western currents along the Ba Lat delta showed that the flow is restored at a distance of approximately 13 km south of the southern spit of the Ba Lat delta. Due to the asymmetrical shape of the Ba Lat delta this reattachment point is expected to be located more south in reality. 3. Alongshore sediment transport modeling by waves only, resulted in southward directed transport in winter, with increasing transport capacities in southern direction. Summer transport directions are north. In the annual transport a point of diverging transport direction has been observed near Van Ly. 4. Lowering of the seabed in front of the dikes resulted in a reduction of sediment transport capacities for all locations. The transport gradients, and hence the eroded volumes, decrease as well. These results lead to the following conclusions: a) The sheltering effect of the Ba Lat delta on the sediment transport in Hai Hau is a reduction in sediment transport capacities in the northern parts during winter monsoon periods. This transport increases in southern direction. During summer monsoon periods waves are not hindered to reach the shore and transport capacities are more constant. b) conclusions regarding the annual erosion were based on the assumption that the large scale residual currents reattach in the middle of Hai Hau. Currents along the coast are expected to increase in southern direction from 0 m/s in the North to 0.2 m/s in the South. c) The present annual eroding volume is approximated at 546.000 m3/year. 4) Lowering of the seabed in front of dikes over the width of the surfzone due to ongoing erosion, reduces the width over which sediment is transported in longshore direction. This also leads to lowering of alongshore sediment transports rates and eroded volumes. An equilibrium situation is not to be expected in the near future.Civil Engineering and Geoscience

Calibration Report Annular Flume: At the Faculty of Coastal Engineering, Zhejiang University

Estuarine systems worldwide contain large quantities of fine (cohesive) sediment. Fate and transport of these sediments is to a large extent governed by erosion and deposition processes. Knowledge on the behaviour of these sediments is required for modeling purposes in order to predict impacts on morphology and environment as a result of e.g. engineering purposes. Physical laboratory experiments are strong instruments to obtain this knowledge under confined conditions. Ongoing research regarding the behavior of sediment from the Yangtze River Estuary is carried out in the framework of the Sino-Dutch cooperation project “Effects of Human Activities on Eco-Morphological processes”. Within this project the State Key Laboratory of Estuarine and Coastal Research (SKLEC) in Shanghai and Delft University of Technology (DUT) share a collaboration partnership. Up to now, sedimentation experiments with Yangtze Estuary sediment are carried out at SKLEC in order to understand its settling and consolidation behavior (Te Slaa et al., 2012). Erosion behaviour is another key process of which its understanding is derides. SKLEC however, doesn’t (yet) have the facilities to perform erosion experiments. Therefore, collaboration was agreed with the Ocean and Hydraulic Engineering Faculty of the Zhejiang University in Hangzhou. This faculty has the availability of an annular flume, which is a powerful tool in determining erosion rates of cohesive sediment. The flume however, has not been in operation for several years and data and knowledge about the flume characteristics is lacking. Therefore a (re-)calibration of this flume is done of which this document describes the procedure and subsequent results. Calibration is carried out with state of the art knowledge on annular flumes, obtain at DUT (Booij, 1994).Hydraulic EngineeringCivil Engineering and Geoscience

Physical model tests with movable bed in front of seadikes

Coastal erosion is a serious problem in several coastal areas in Vietnam. One of these areas is the district of Hai Hau, located in Nam Dinh province. This district suffers from severe coastal erosion, the coastline has been retreating for more than 100 years. Efforts have been made to stop this erosion which resulted in the construction of strong seadikes along almost the Hai Hau coastline. These seawalls are placed relatively close to the waterline. Therefore, during extreme conditions the waterlevels in front of the dike become high. Field observations have shows that during these extreme conditions scour holes are formed in front of these dikes. Since little is know about the influence of the geometrical dike properties on the scour development wave flume test have been carried out with a movable bed to gain more insight in this problem. These tests have been carried out in the Holland wave flume of the Hanoi Water Resources University. In addition the tests were simulated in the computational model DUROSTA. This model is developed to determine dune erosion under storm conditions. The model is extended so that it can include hard structures such as seadikes. The geometrical dike properties that are tested are: Dikeslopes. - Initial slopes in the seabed near the dikes. - Cylinders placed at the toe of the dikes. - Crownwalls placed on top of the dikes. The most important results following from this series of test are the influence of the reflection coefficient of the scour behavior. In the wave flume tests the two properties that are of main positive influence on the reflection coefficient are: Increasing dike slope. - Increasing wave period. - Increasing crownwall height. - An increasing reflection coefficient does not give larger scour depths as is to be expected from known literature about scour near seawalls. The area and length of erosion however does become larger. The influence of crownwalls on the reflections coefficient is governing over the influence of increasing wave period on the reflection coefficient. An other test properties that is of influence for a larger area of erosion is the initial sloping seabed. The modeling of the wave flume tests with DUROSTA give the following results: The performance of DUROSTA after 2 hours is quite accurate with the measured field experiments, DUROSTA underestimates the scour depth with an average of 4%. The average underestimation of the scour depth in DUROSTA after 4 hours of testing is 19%. In DUROSTA the influence of the wave period on the scour development is governing over the influence of increasing crownwall height.Civil Engineering and Geoscience

Deposition and erosion of silt-rich sediment-water mixtures

From a granulometric point of view, sediment can be classified as sand, silt and clay. Silt is thereby defined as sediment with particle sizes equal to, or larger than 2 μm and smaller than 63 μm, with quartz or feldspar as base mineral. Note that quartz and feldspar particles can be smaller than 2 μm. To date, our knowledge on deposition and erosion processes of silt and siltrich sediment-water mixtures is small compared to their sandy or clayey counterparts, hampering our understanding of large-scale morphological behaviour of silt-dominated systems. The most important difference between clay and silt is that clay consists of clay minerals which have cohesive properties, and as a result, the erosion and deposition of clay beds is influenced by cohesion. The behaviour of cohesive sediment in suspension and in the bed is influenced by flocculation, permeability, effective stress and rheological properties which are related to electro-chemical properties of the base minerals. Silt particles do not have cohesive properties, but there are indications that their erosion behaviour can be apparently cohesive (Roberts et. al., 1998, Van Maren et al., 2009a). Permeability effects are likely to play a role in the behaviour of silt-water mixtures due to the small particles sizes of silt. Such effects result from a difference in timescales between the forcing and the response of the bed and result in apparently cohesive behaviour. Examples are the development of the bed strength with increasing hydrodynamic forcing or the dissipation of overpressures within a compacting silt bed. Such behaviour is characteristic for cohesive material and not for granular material such as silt, and is referred to as apparently cohesive behaviour. In existing literature, the physical processes that control the behaviour of silt are mostly described qualitatively, and silt-specific formulations for hindered settling, compaction and erosion do not exist. Through this thesis, quantitative insights into the physical processes controlling the behaviour of silt-water mixtures have been derived. The overall objectives of this thesis were to i) determine the hindered settling behaviour of silt-water mixtures, ii) determine the deposition and compaction behaviour of silt beds, and iii) determine the erosion behaviour of silt beds.Environmental Fluid Mechanic

On the hindered settling of silt-water mixtures

The sedimentation behavior of silt particles in the hindered settling regime has been considered. Therefore, laboratory experiments are carried in especially designed settling columns. Silt-water mixtures are prepared and allowed to settle at various initial concentrations. During the settling process, a continuous vertical concentration profile is measured. Settling velocities are obtained from these profiles and are compared with the widely used Richardson and Zaki (1954) expression. This comparison showed that the Richardson and Zaki expression underestimates the settling velocities for the finest silts at high concentrations. For the coarsest silt the Richardson and Zaki expression performed will with the measured data. We anticipate that this is the effect of the particle size on the apparent viscosity of the settling silt-water mixture.Hydraulic EngineeringCivil Engineering and Geoscience